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Show 446 MODIFIED CIRCUl\1NUTATION. C'IIAP. Ylll. a hright sun, they stoocl at 56° a hove tho horizon; they were th en protected from the rays of the sun, lmt were loft well illuminated from above, and after 30m. they had fa llen 40°, for they now stood at only 16° aboYe the horizoll. ome young plants of Phas~'olus l!m·na"d"sii hacl heen oxpoF>erl io tho amo bright sunlight, and their br oacl, unifohate, li1·. t leaves now stood up almost or quite vertically, as did many of tho leaflets on the trifoliate secondary leaves ; but some of the leaGcts bnd twisted round on th eir own axes by as mnch aR 90° without rising, so as to present their edges to the sun. 'fhc leaflets on the same leaf sometimes behaved in these two different manners, hut always with the r esult of being less intensely illuminated. These plants were then protected from the sun, and were looked at after 1} h.; and now all the leaves and leaflets had re· assumed their ordinary sub-horizontal positions. The copper· colomod cotyledons of some seedlings of Cassia mimosoides were horizontal in tho moming, hut a fter the sun had shone on them each had risen 45 ~ 0 above the hori7.on. Tlw movement in th~se several cases must not be confounded with the sudden closing of the leaflets of M£m()sa pudica, which may sometimes be noticed when a plant which has been kept in an obs~ure place is suddenly exposed to tho sun; for in thi.· case the l1ght seems to act, as if it were a touch. From Prof. Wiesner's interesting observations, it is proba~le that the above movements have been acquired for a spec1al pm·pose. The chlorophyll in loaves is often ii~ju.red by too intense a light, and Prof. Wiesner * believes that Jt 1s protcc~ed by the most diversified means, such as the preRence of han·s, colouring matter &c. and amongst other means by the leaves presenting their ' edge's to the sun, EiO that t J1 e bla d e s then receive much less light. He experimented on the young leaflets of Robinia, by fixing them in such a position. that they co.nlrl not escape being intensely illuminated, wh1lst others were allowed to place themselves obliquely ; and the former began to suffer from the light in tho course of two days. d Jn the cases above gi·v en, the lea iie t s move either u}JWill' s * 'Die Niiturlichen Einrichtungen zum Schutze des Chlorophylls,' &c., 1876. Pringsheim has recently observed under tho microscope the destruction of chlorophyll in a few minutes by the action of concentrated Ji"b}· from the sun, in the presencot oxygen. See, also, Stahl on t 10 protection of ('hloroph,vll. frO:, intense light, in ' Bot. 7,0ltuuo, 1880. CHAP. VIII. P ARAHELIOTROPISM. 4-17 or twist laterally, so as to place their edges in the direction of th ~ sun's light; but Cohn long ago obser ved that the leaflets of Oxalis bend downwards when fully ?XIJO ed to th? sun. \Ve witnessed a striking instance of this movement m tho very large leaflets of U. Ort~gesii. A similar movement ~~y !rcquently be observed With the leafi~t~ of ..A ve1·rhoa b~hmbt .C a member of the Oxalidre); and a leaf 1s hc1·e r ep1·esonted (Fl O'. 180) on which tho sun had shone. A diagram (Fig. 134) was given in the last chapter, representing th.o oscillations by which a leaflet rapidly descended under those circumstances ; ancl tho movement may be been closely to resemble that (Fig. 133) by Fi !\. 180. Averrhoa bilimbi: leaf with leaflets depressed after exposure to sunshin e ; but the leaflets are sometimes more depressed than is her e shown. Figure much reduced. which it assumed its nocturnal position. It is an interesting fact in relation to our present subject that, as Prof. Batalin informs u.s in a letter, dated February, 1879, tho leaflets of lkalis acetosella may be dnily exposed to tho sun during many weeks, and they do not suffer if they arc allowed to dApress themselves; but if this be prevented, they lose their colour and wither in two or three days. Yet the duration of a leaf is about two months, when subjected only to diffused light; and in this case the leaflets never sink downwards during the day. As the upward movements of the leaflets of Robinia, and the downward movements of those of Oxalis, have ~en proved to be highly beneficial to these plants wnen subjected to bright sunshine, it seems probable that they have been acquired for the special purpos"' of avoiding too intense an illumination. As it would have been very troublesome in all the above cases to |